Novel process for acyclic phosphonate nucleotide analogs

ABSTRACT

The present invention provides a novel process for the preparation of acyclic phosphonate nucleotide analogs using novel intermediates. Thus, for example, (R)-9-(2-phosphonomethoxypropyl)adenine is reacted with dimethylformamide dimethylacetal to give N 4 -dimethylaminomethyledino-9-(2-phosphonomethoxy ethyl) adenine, which is then reacted with chloromethyl-2-propyl carbonate in presence of triethylamine to give (R)—N 4 -Dimethylaminomethyledino-9-(2-phosphono methoxypropyl) adenine disoproxil, followed by deprotection with acetic acid to get tenofovir disoproxil. Tenofovir disoproxil is then treated with fumaric acid to give tenofovir disoproxil fumarate.

FIELD OF THE INVENTION

The present invention provides a novel process for the preparation ofacyclic phosphonate nucleotide analogs using novel intermediates.

BACKGROUND OF THE INVENTION

Acyclic nuleoside phosphonates (ANPs) represent a class of nucleotideanalogues in which a phosphonate group is linked to the alkyl side chainof various purines and pyrimidines. This class of nucleoside analoguespossesses broad-spectrum antiviral activity, together with a high levelof selectivity in vitro and in vivo. Among them adefovir chemically9-(2-phosphonylmethoxyethyl)adenine and its ester derivatives (describedin U.S. Pat. No. 4,808,716), tenofovir chemically(R)-9-(2-phosphonomethoxypropyl) adenine and its ester derivatives(described in U.S. Pat. No. 5,922,695) are having potent and selectiveactivity against HIV and other retroviruses such as retro-, herpes- andhepadnaviruses.

Tenofovir disoproxil is represented by the following structure offormula 1:

Adefovir dipivoxil is represented by the following structure of formula2:

These compounds are normally prepared by the esterification ofphosphonic acid of formula 3:

wherein R′ is alkyl or H;with an appropriate halide to obtain the compounds such as those shownin formula 2 and 3.

It has been found that introduction of groups such asisopropyloxycarbonyloxy methyl or pivaloyloxy methyl group to phosphonicacid compounds of formula 3 for the preparation of the phosphonic acidbis ester compounds such as tenofovir disoproxil of formula 1 oradefovir dipivoxil of formula 2 is associated with the problem ofalkylation of N⁴-amine group to form corresponding N-alkylatedimpurities. Excess use of alkylating agents leads to the formation ofN⁴-alkylated impurity. Similarly the use of lower quantities ofalkylating agents leads to lower yields of products like tenofovirdisoproxil and adefovir dipivoxil and formation of higher amounts of thecorresponding mono ester impurities.

In an effort to solve the above said problems, we have succeeded infinding protecting agents that are advantageous in that a) the saidprotecting agents can successfully protect amino group and at the sametime do not form esters with phosphonic acid group of the compound offormula I:

b) when the amine group is once protected with these protecting groups,then desired esterification can be carried out; and c) then the saidprotecting group can be removed under conditions that does not hydrolyseor solvolyse the ester groups.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a processfor preparing the compound of formula I:

wherein R₁ is C₁ to C₃-alkyl or H; and R₂ is H or R—C(O)—OCH₂— wherein Ris C₁ to C₆-alkyl or C₁ to C₆-alkoxide; provided both the R₂ are not H;

or a pharmaceutically acceptable salt thereof:

which comprises:

-   a) reacting the compound of formula II:

wherein R₁ is same as defined above

with an acetal of formula III:

wherein R₃ is independently C₁ to C₆-alkyl or —H, wherein the said alkylis unsubstituted or is substituted with 1 or 2 halo, cyano, nitro;

R₄ is H or C₁ to C₆-alkyl, wherein the said alkyl is unsubstituted or issubstituted with 1 or 2 halo, cyano, nitro;

R₅ is C₁ to C₆-alkyl, wherein the said alkyl is unsubstituted or issubstituted with 1 or 2 halo, cyano, nitro;

to obtain the compound of formula IV:

wherein R₁, R₃ and R₄ are same as defined above;

-   b) reacting the compound of formula IV with the compound of formula    V:

R—C(O)—O—CH₂—Y  V

wherein R is same as defined above, Y is a leaving group to formprotected compound of formula VI:

wherein R₁, R₂, R₃ and R₄ are same as defined above;

-   c) deprotecting the compound of formula VI to form the compound of    formula I; and-   d) optionally converting the compound of formula I to a    pharmaceutically acceptable salt.

The compounds of formula IV and VI are novel and form part of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, there is provided a processfor preparing the compound of formula I:

wherein R₁ is C₁ to C₃-alkyl or H; and R₂ is H or R—C(O)—OCH₂— wherein Ris C₁ to C₆-alkyl or C₁ to C₆-alkoxide; provided both the R₂ are not H;or a pharmaceutically acceptable salt thereof using novel intermediates.

The process of the invention is described herein under.

The compound of formula II:

wherein R₁ is same as defined for formula I;

is reacted with an acetal of formula III:

wherein R₃ is independently C₁ to C₆-alkyl or —H, wherein the said alkylis unsubstituted or is substituted with 1 or 2 halo, cyano, nitro;

R₄ is H or C₁ to C₆-alkyl, wherein the said alkyl is unsubstituted or issubstituted with 1 or 2 halo, cyano, nitro;

R₅ is C₁ to C₆-alkyl, wherein the said alkyl is unsubstituted or issubstituted with 1 or 2 halo, cyano, nitro;

to obtain the compound of formula IV:

wherein R₁ is as defined for formula I; R₃ and R₄ are as defined forformula III.

The reaction of the compound of formula II with an acetal of formula IIIis carried out in any inert solvent. Preferably the reaction is carriedout in N,N-dimethylformamide.

The reaction may be carried out at below the boiling temperature of thesolvent used, preferably at 20° C. to boiling temperature of the solventused.

The compound of formula IV obtained above can be used directly in thenext step; or can be isolated from the reaction mass and then used inthe next step.

The compound of formula IV is reacted with the compound of formula V:

R—C(O)—O—CH₂—Y  V

wherein R is same as defined for formula I, Y is a leaving group toobtain protected compound of formula VI:

wherein R₁ and R₂ are as defined for formula I, and R₃ and R₄ are asdefined for formula III.

Preferably R is isopropoxy- or tert-butyl, and the leaving group Y ishalo or sulfonyloxy. Preferable halo group is chloro or bromo and morepreferable halo group is chloro.

Preferably the sulfonyloxy group is methane sulfonyloxy, benzenesulfonyloxy, o, m or p-toluene sulfonyloxy or m-nitrobenzene sulfonyloxygroup.

Preferably both the R₂ groups are R—C(O)—OCH₂—.

Preferably, more than two moles of the compounds of formula V are usedper mole of the compounds of formula IV to obtain the compound VI,wherein both the R₂ groups are R—C(O)—OCH₂—, more preferably, 3 to 15moles, still more preferably 3 to 10 moles of the compounds of formula Vare used per mole of the compound of formula IV.

The reaction may be carried out in any suitable solvent and may becarried out at 10° C. to boiling temperature of the solvent used.Preferable solvent is selected from ester solvents such as ethylacetate, methyl acetate, isopropyl acetate, tert-butyl acetate, ethylformate and methyl formate; N,N-dimethyl formamide;N,N-dimethylacetamide; dimethyl sulfoxide; cyclic amide solvents such asN-methyl-2-pyrrolidinone; dioxane; tetrahydrofuran; ketonic solventssuch as acetone, diethyl ketone, methyl ethyl ketone, methyl isobutylketone and methyl propyl ketone; chlorinated hydrocarbon solvents suchas methylene chloride, chloroform, carbon tetrachloride and ethylenedichloride; hydrocarbon solvents such as benzene, toluene, xylene,hexane and cyclohexane; and ether solvents such as diethyl ether,diisopropyl ether and tert-butyl methyl ether. More preferable solventis selected from N-methyl-2-pyrrolidinone, N,N-dimethylformamide,N,N-dimethylacetamide and tetrahydrofuran. Most preferable solvent isN-methyl-2-pyrrolidinone or N,N-dimethylformamide.

The reaction is preferably carried out in the presence of a base such asan amine base.

The compound of formula VI may preferably be crystallized from thereaction mass or the compound of formula VI may be used directly in thenext step.

The compound of formula VI is subjected to deprotection to obtain thecompound of formula I.

The deprotection may be carried out using for example carboxylic acidssuch as formic acid, acetic acid or sulfonic acids such as methanesulfonic acid, toluene sulfonic acid, water or a mixture thereof. Thereagent used for deprotection is not critical so far as phosphonic estergroup is not hydrolysed or solvolysed.

The compound of formula I is optionally converted to thepharmaceutically acceptable salts by known methods.

R₃ in the formulae III and IV are preferably independently C₁ toC₆-alkyl, more preferably methyl or ethyl and still more preferablymethyl.

R₄ in the formulae III, IV and VI are preferably —H.

R₅ in the formula III is preferably methyl or ethyl.

R₁ in the formulae I, II, IV and VI are preferably methyl or H.

If R₁ is alkyl, the compound of formula II, IV, VI and I are representenantiomers and mixture of enantiomers and come under the scope of theinvention.

The preferred compounds of formula I prepared according to the presentinvention are the compounds of formula I wherein R is isopropoxy ortert-butyl and R₁ is H or methyl. Preferable compounds of formula I are

-   i) (R)-9-(2-phosphonomethoxypropyl) adenine    bis(isopropyloxycarbonyloxy methyl) ester (tenofovir disoproxil) and-   ii) 9-(2-phosphonylmethoxyethyl)adenine di(pivaloyloxymethyl) ester    (adefovir dipivoxil).

The preferred compounds of formula IV are the compounds of formula IVwherein R₁ is methyl or H; R₃ is methyl or ethyl, and R₄ is H.

Preferable compounds of formula VI are the compounds of formula VIwherein R₃ is methyl or ethyl, R₄ is H; R₁ is H or methyl and R isisopropoxy or tert-butyl.

The invention will now be further described by the following examples,which is illustrative rather than limiting.

EXAMPLE 1 Step-I

(R)-9-(2-phosphonomethoxypropyl)adenine (14 gm) is dissolved indimethylformamide (60 ml) and dimethylformamide dimethylacetal (21 gm)is added slowly at 25-30° C. for 10 minutes. The temperature is raisedto 55° C. and maintained for 3 hours at 50-55° C. Then the solvent isdistilled under vacuum at 75-80° C. to give 20 gm of(R)—N⁴-Dimethylaminomethyledino-9-(2-phosphonomethoxypropyl)adenine asresidue.

Step-II

To the mixture of (R)—N⁴-Dimethylaminomethyledino-9-(2-phosphonomethoxypropyl)adenine (20 gm), N-methyl-2-pyrrolidinone (80 ml) andtriethyl amine (20 ml), chloromethyl-2-propyl carbonate (30 gm) is addedslowly at 25-30° C. for 30 minutes, the temperature is raised to 60° C.and maintained for 4 hours at 55-60° C. Then the reaction mass is cooledto 25-30° C. Ethyl acetate (260 ml) is added and stirred for 20 minutesat 25-30° C. Filtered the salts on hi-flo bed and washed the ethylacetate layer with water (140 ml), then washed with 10% sodium chloride(100 ml) and then the resulting organic layer is distilled under vacuumto give 20 gm of (R)—N⁴-Dimethylaminomethyledino-9-(2-phosphonomethoxypropyl) adenine bis(isopropyloxycarbonyloxymethyl) ester.

Step-III

(R)—N⁴-Dimethylaminomethyledino-9-(2-phosphonomethoxypropyl) adeninebis(isopropyloxycarbonyloxymethyl) ester (10 gm) is added to 80% aceticacid (80 ml) at 25-30° C., the temperature is raised to 50-55° C. andmaintained for 3 hours at the same temperature. Then acetic acid isdistilled under vacuum at 70° C. Water (40 ml) and ethyl acetate (130ml) are added to the residue, pH of the solution is adjusted to 7.0 withtriethylamine and then separated the layers. The resulting organic layeris washed with 10% sodium chloride solution (35 ml) and distilled undera vacuum to give 8 gm of tenofovir disoproxil (HPLC Purity: 99.2%).

Step-IV

Tenofovir disoproxil (2.5 gm) is dissolved in isopropyl alcohol (12.5ml), fumaric acid (0.6 gm) is added at 25-30° C. and then thetemperature is raised to 45° C., maintained for 20 minutes. The reactionmass is cooled to 25-30° C. and stirred for 1 hour at the sametemperature and again mass is cooled to 0-5° C., stirred for 1 hour at0-5° C. Filtered the material, washed with isopropyl alcohol (2 ml) andfinally washed with hexane to give 2.4 gm of tenofovir disoproxilfumarate (HPLC Purity: 99.8%).

EXAMPLE 2 Step-I

9-(2-Phosphonomethoxyethyl)adenine (20 gm) is dissolved in dimethylformamide (80 ml) and dimethylformamide dimethylacetal (30 gm) is addedslowly at 25-30° C. for 10 minutes. The temperature is raised to 55° C.and maintained for 3 hours at 50-55° C. Then the solvent is distilledunder vacuum at 75-80° C. to give 28 gm ofN⁴-dimethylaminomethyledino-9-(2-phosphono methoxyethyl) adenine asresidue.

Step-II

To the mixture of N⁴-dimethylaminomethyledino-9-(2-phosphonomethoxyethyl) adenine (25 gm), N-methyl-2-pyrrolidinone (100 ml) andtriethylamine (38 gm), chloromethyl pivalate (57 gm) is added slowly at25-30° C. for 30 minutes, the temperature is raised to 45° C. andmaintained for 19 hours at 40-45° C. Then the reaction mass is cooled to25-30° C. and filtered the salts on hi-flo bed. To the reaction massethyl acetate (300 ml) is added and stirred for 20 minutes at 25-30° C.The ethyl acetate layer is washed with water (175 ml) and then washedwith 10% sodium chloride solution (125 ml). The resulting organic layeris distilled under vacuum to give 30 gm ofN⁴-dimethylaminomethyledino-9-(2-phosphonomethoxy ethyl)adininedi(pivaloyloxymethyl) ester.

Step-III

N⁴-Dimethylaminomethyledino-9-(2-phosphonomethoxy ethyl) adininedi(pivaloyloxymethyl) ester (10.5 gm) is added to 80% acetic acid (80ml) at 25-30° C., the temperature is raised to 50-55° C. and maintainedfor 3 hours at the same temperature. Then acetic acid is distilled undervacuum at 70° C. Water (50 ml) and ethyl acetate (150 ml) are added tothe residue, pH of the solution is adjusted to 7.5 with triethylamineand then separated the layers. The resulting organic layer is washedwith 10% sodium chloride solution (50 ml), distilled under vacuum andthe residue obtained is crystallized from acetone/diisopropyl ether togive 8 gm of adefovir dipivoxil (HPLC Purity: 99.6%).

1. A process for preparation of the compound of formula I:

wherein R₁ is C₁ to C₃-alkyl or H; and R₂ is H or R—C(O)—OCH₂— wherein Ris C₁ to C₆-alkyl or C₁ to C₆-alkoxide; provided both the R₂ are not H;or a pharmaceutically acceptable salt thereof: which comprises: a)reacting the compound of formula II:

wherein R₁ is same as defined above with an acetal of formula III:

wherein R₃ is independently C₁ to C₆-alkyl or —H, wherein the said alkylis unsubstituted or is substituted with 1 or 2 halo, cyano, nitro; R₄ isH or C₁ to C₆-alkyl, wherein the said alkyl is unsubstituted or issubstituted with 1 or 2 halo, cyano, nitro; R₅ is C₁ to C₆-alkyl,wherein the said alkyl is unsubstituted or is substituted with 1 or 2halo, cyano, nitro; to obtain the compound of formula IV:

wherein R₁, R₃ and R₄ are same as defined above; b) reacting thecompound of formula IV with the compound of formula V:R—C(O)—O—CH₂—Y  V wherein R is same as defined above, Y is a leavinggroup to form protected compound of formula VI:

wherein R₁, R₂, R₃ and R₄ are same as defined above; c) deprotecting thecompound of formula VI to form the compound of formula I; and d)optionally converting the compound of formula I to a pharmaceuticallyacceptable salt.
 2. The process as claimed in claim 1, wherein R₃ isindependently C₁ to C₆-alkyl.
 3. The process as claimed in claim 2,wherein R₃ is methyl or ethyl.
 4. The process as claimed in claim 3,wherein R₃ is methyl.
 5. The process as claimed in claim 1, wherein R₄is H; R₅ is methyl or ethyl and R₁ is methyl or H.
 6. The process asclaimed in claim 5, wherein R₅ is methyl and R₁ is methyl.
 7. Theprocess as claimed in claim 1, wherein the reaction in step (a) iscarried out in an inert solvent.
 8. The process as claimed in claim 7,wherein the solvent is N,N-dimethyl formamide.
 9. The process as claimedin claim 1, wherein the reaction in step (a) is carded out at below theboiling temperature of the solvent used.
 10. The process as claimed inclaim 9, wherein the reaction is carried out at 20° C. to boilingtemperature of the solvent used.
 11. The process as claimed in claim 1,wherein the R in formula V is isopropoxy- or tert-butyl, and the leavinggroup Y is halo or sulfonyloxy.
 12. The process as claimed in claim 11,wherein the halo group is chloro or bromo.
 13. The process as claimed inclaim 12, wherein the halo group is chloro.
 14. The process as claimedin claim 12, wherein the sulfonyloxy group is methane sulfonyloxy,benzene sulfonyloxy, o, m or p-toluene sulfonyloxy or m-nitrobenzenesulfonyloxy group.
 15. The process as claimed in claim 1, Wherein boththe R₂ groups in formula VI are R—C(O)—OCH₂—.
 16. The process as claimedin claim 1, wherein the reaction in step (b), more than two moles of thecompounds of formula V are used per mole of the compounds of formula IVto obtain the compound VI.
 17. The process as claimed in claim 16,wherein 3 to 15 moles of the compounds of formula V are used per mole ofthe compounds of formula IV.
 18. The process as claimed in claim 17,wherein 3 to 10 moles of the compounds of formula V are used per mole ofthe compounds of formula IV.
 19. The process as claimed in claim 1,wherein the reaction in step (b) is carried out in a suitable solventand is carried out at 10° C. to boiling temperature of the solvent used.20. The process as claimed in claim 19, wherein the solvent is selectedfrom ester solvents such as ethyl acetate, methyl acetate, isopropylacetate, tert-butyl acetate ethyl formate and methyl formate;N,N-dimethylformamide; N,N-dimethylacetamide; dimethyl sulfoxide; cyclicamide solvents such as N-methyl-2-pyrrolidinone; dioxane;tetrahydrofuran; ketonic solvents such as acetone, diethyl ketone,methyl ethyl ketone, methyl isobutyl ketone and methyl propyl ketone;chlorinated hydrocarbon solvents such as methylene chloride, chloroform,carbon tetrachloride and ethylene dichloride; hydrocarbon solvents suchas benzene, toluene, xylene, hexane and cyclohexane; and ether solventssuch as diethyl ether, diisopropyl ether and tert-butyl methyl ether.21. The process as claimed in claim 20, wherein the solvent is selectedfrom N-methyl-2-pyrrolidinone, N,N-dimethylformamide,N,N-dimethylacetamide and tetrahydrofuran.
 22. The process as claimed inclaim 21, wherein the solvent is N-methyl-2-pyrrolidinone orN,N-dimethylformamide.
 23. The process as claimed in claim 1, whereinthe reaction in step (b) is carried out in presence of a base.
 24. Theprocess as claimed in claim 23, wherein the base is an amine base. 25.The process as claimed in claim 1, wherein the deprotection in step (c)is carried out using carboxylic acids such as formic acid, acetic acidor sulfonic acids such as methane sulfonic acid, toluene sulfonic acid,water or a mixture thereof.
 26. The process as claimed in claim 1,wherein the compounds of formula I prepared are the compounds of formulaI wherein R is isopropoxy or tert-butyl and R₁ is H or methyl.
 27. Theprocess as claimed in claim 26, wherein the compound of formula 1 is i)(R)-9-(2-phosphonomethoxypropyl) adenine bis(isopropyloxycarbonyloxymethyl) ester (tenofovir disoproxil) or ii)9-(2-phosphonylmethoxyethyl)adenine di(pivaloyloxymethyl) ester(adefovir dipivoxil).
 28. Compound of formula IV:

wherein R₁ is C₁ to C₃-alkyl or H; R₃ is independently C₁ to C₆-alkyl or—H, wherein the said alkyl is unsubstituted or is substituted with 1 or2 halo, cyano, nitro; and R₄ is H or C₁ to C₆-alkyl, wherein the saidalkyl is unsubstituted or is substituted with 1 or 2 halo, cyano, nitro.29. The compound as claimed in claim 28, wherein R₁ is methyl or H; R₃is methyl or ethyl; and R₄ is H. 30.(R)—N⁴-Dimethylaminomethyledino-9-(2-phosphonomethoxypropyl) adenine.31. N⁴-dimethylaminometyledino-9-(2-phosphonomethoxyethyl) adenine. 32.Compound of formula VI:

wherein R₁ is C₁ to C₃-alkyl or H; R₂ is H or R—C(O)—OCH₂— wherein R isC₁ to C₆-alkyl or C₁ to C₆-alkoxide; provided both the R₂ are not H; R₃is independently C₁ to C₆-alkyl or —H, wherein the said alkyl isunsubstituted or is substituted with 1 or 2 halo, cyano, nitro; and R₄is H or C₁ to C₆-alkyl, wherein the said alkyl is unsubstituted or issubstituted with 1 or 2 halo, cyano, nitro.
 33. The compound as claimedin claim 31, wherein R₃ is methyl or ethyl, R₄ is H, R₁ is H or methyland R is isopropoxy or tert-butyl. 34.(R)—N⁴-Dimethylaminomethyledino-9-(2-phosphonomethoxypropyl)adeninebis(isopropyloxycarbonyloxymethyl) ester. 35.N⁴-dimethylaminomethyledino-9-(2-phosphonomethoxyethyl)adininedi(pivaloyloxymethyl) ester.